For a trap-free single layer single carrier (hole-only or electron-only) organic device of thickness d, dielectric constant εr, and mobility μ, the relation of current density j with voltage V is described by the well-known Mott–Gurney equation, [Formula: see text], which can be rewritten as [Formula: see text], where [Formula: see text] can be called space-charge limited (SCL) conductance. We investigated the current–voltage characteristics of vertical organic heterojunction phototransistors based on graphene as the source and C60/copper phthalocyanine (CuPc) heterojunction as the photoactive layer. We found that the drain current vs drain voltage ( Id- Vd) characteristic is composed of two distinct SCL conductions with SCL-conductance being strongly dependent on the gate voltage and illumination intensity. At low gate voltages, the Id- Vd curve can be divided into two sections of SCL current conduction with different SCL-conductance: the lower conductance in the lower drain voltage range and the higher conductance in the higher drain voltage range. Both low and high SCL-conductance increase with the gate voltage and illumination intensity. However, as the gate voltage increases to a certain threshold, the two SCL sections unify to only one with the conductance being between them. Our findings implicate that the current conduction of an ideal vertical organic phototransistor (VOPT), whose source/organic interface contact is Ohmic and organic semiconductor is trap free, can be well modeled by the SCL conduction theory with carrier density dependent mobility, which is strongly related to the gate voltage and illumination intensity, and that the mobility of VOPTs at a certain gate voltage can be extracted by the Mott–Gurney equation.

including the bandwidth, photoresponsivity, and photoconductive gain. [5,6] To overcome these limitations, graphene hybrid photodetectors incorporating photo sensitive materials such as single crystals, [7] 2D materials, [8] organic polymers, [9] colloidal quantum dots, [10,11] and silicon [12] have been widely adopted to enhance the photoresponse. For example, methylammonium triiodideplumbate (CH 3 NH 3 PbI 3 ) perovskite incorporated with graphene yielded a planar heterojunction photodetector with a broad spectral response from 405 to 800 nm. [13] Moreover, a hybrid graphene phototransistor adopting organic semiconductor poly (3-hexylthiophene) (P3HT) achieved a high photoresponse up to 10 5 A W −1 . [9] Despite that the combination of photosensitive layers and graphene has proved to be an effective strategy to enhance the performance of phototransistors, [14][15][16][17][18] the absence of an internal built-in electric field using a single type of light-sensitive layers leads to relatively low photoresponsivity for photodetection.Recently, the combination of a PN heterojunction into graphene optoelectronic devices as the absorption layer has been considered as an emerging means to enhance the responsivity and photoconductive gain. [19,20] The internal built-in electric field within the PN heterojunction enables efficient dissociation for excitons. [19] Meanwhile, different absorption wavelengths of the PN layers with proper band alignment can lead to unique bidirectional photoresponse, enabling the extension of the photo detection bandwidth. [21] Recently, a pentacene planar heterojunction was employed to integrate with graphene to fabricate a high-performance transistor, showing bidirectional photoresponse at different wavelength regions with an ultra-wide spectrum detection from 405 to 1550 nm. [22] Meanwhile, a vertical graphene-C 60 -graphene heterojunction in a 250 × 250 photodetector array was assembled to realize a high photoresponsivity of 3.4 × 10 5 A W −1 (λ = 405 nm, i.e., visible) with bidirectional photocurrent responses. [23] In spite of the considerable progress made in the last decades to achieve highperformance detection for visible and ultraviolet light, most of the graphene phototransistors are still limited to low photoresponsivity at the near-infrared (NIR) region. Graphene-organic heterojunction phototransistorshave great potential to achieve sensitive photoresponse owing to the excellent absorption of organic layers and fast charge transport in graphene. However, the photoresponse of most graphene-based phototransistors is limited within visible light region with narrow bandwidth and poor sensitivity in the near-infrared (NIR) region. Herein, a graphene-organic NIR phototransistor is fabricated by integrating an organic heterojunction layer composing of phthalocyanine molecules and fullerene C 60 onto the graphene channel. The phototransistor exhibits a high photoresponsivity of 2.2 × 10 3 A W −1 under 850 nm irradiation with the power density of 35.4 mW cm −2 (V ds = 1 V). Meanwhile, a ...

We propose a time-delayed photonic reservoir computing (RC) architecture utilizing a reflective semiconductor optical amplifier (RSOA) as an active mirror. The performance of the proposed RC structure is investigated by two benchmark tasks, namely the Santa Fe time-series prediction task and the nonlinear channel equalization task. The simulation results show that both the prediction and equalization performance of the proposed system are significantly improved with the contribution of RSOA, with respect to the traditional RC system using a mirror. By increasing the drive current of the RSOA, the greater nonlinearity of the RSOA gain saturation is achieved, as such the prediction and equalization performance are enhanced. It is also shown that the proposed RC architecture shows a wider consistency interval and superior robustness than the traditional RC structure for most of the measured parameters such as coupling strength, injection strength, and frequency detuning. This work provides a performance-enhanced time-delayed RC structure by making use of the nonlinear transformation of the RSOA feedback.